Thirty-four years ago, a brilliant young hedge-fund manager helped engineer a $1 billion payday for his employer by betting a major currency would be devalued. He made a similar killing two decades later. Now, Scott Bessent finds himself on the other side of the foreign-exchange trenches as U.S. Treasury Secretary, and it’s the sagging yen he’s fighting to defend, not the dollar.

How he does it matters to investors at home too. Even speculators with the deepest pockets are veritable Davids when facing government Goliaths, but they have one advantage. It can get more awkward for a country to artificially strengthen a currency than to weaken it.

When Bessent helped George Soros break the pound on “Black Wednesday” in 1992, he reckoned that the country’s floating-rate mortgages made it politically painful for Britain to raise interest rates to prop it up. Too painful, it turned out.